Diesel engines emit a hazardous, sooty exhaust that can be rendered less hazardous by using diesel particulate filters. The soot trapped by such filters builds up over time, requiring periodic regeneration (i.e., removal of the entrapped soot).
Two techniques for regenerating diesel particulate filters predominate. One technique involves the periodic release of a burning gas. The other technique utilizes electrical heating elements in contact with the filtering element. An example of the latter technique is disclosed in Offenlegungsschrift No. DE 38 00 723 (Heuer et al.), laid open Jul. 27, 1989, wherein the regenerable particulate filter trap has at least one filter element arranged in a filter housing, with a wire heating device on the exterior of the filter element.
European Pat. Appl. No. 0 275 372 (Gurtler et al.), laid open Jan. 3, 1990, discloses a soot filter having a heating element located on the interior or exterior surface of the filter element, where the heating element can consist of crossed wires, expanded metal, or a perforated metal plate.
U.K. Pat. Appln. GB 2193656 (Henkel), laid open Feb. 17, 1988, discloses a regenerable diesel particulate trap wherein the filter elements are helically wound with two wires which are continuously maintained under an electrical potential. When carbon particles (i.e., soot) buildup on the filter element, such that the conductivity gap between the two wires is closed, electric current flows through the resisting carbon particles causing them to heat up to the point where they ignite.